This poem is a homage to Wordsworth.  To a great extent, the Romantic movement is seen in two phases.  Wordsworth and Coleridge represented the first phase of the movement.  Leaders and the voices that articulated it in terms of what it was designed to be and its revolutionary expression of the good, both thinkers wrote extensively on the subject in both prose and poetic forms.  The second part of the movement was evident in the Shelley, Byron, and Keats triad.  This poem represents the second paying honor to the first.  The first two lines of the poem reflect an appreciation of what Wordsworth meant and the themes in which he articulated zealous support for in his poetry.  There is an overriding sense of loss in the poem, as Shelley speaks of Wordsworth and his loves as things of the past.  This is seen in lines such as, "Thou wert a lone star whose light did shine."  The closing of the poem brings to light that there will not be another one like Wordsworth on the scene for some time, with Shelley fully recognizing it.  The theme of this reading would be an honor for those who came in front of Shelley and respect for what others did.  Certainly, Shelley would agree with the notion that "we stand on the shoulders of giants," and one can see this honoring as such.  There could be an alternate reading of this, though.  Shelley is a poet who is as obsessed with Romanticism as he is with staking his own claim to artistic immortality and the notion of being regarded past his time.  Poems like "Ode to the West Wind" and "Ozymandias" help to bring this to light.  It is not entirely inconceivable that Shelley might have wished to usurp Wordsworth's role as leader of the Romantic movement and seek to become its leader himself.  It is not entirely foreign to see Shelley as wishing to argue that the younger voices such as Byron or Keats and his own are the new leaders of the movement.  In this light, the poem might serve as a death knell to Wordsworth and Coleridge and a signal to usher in the new leaders of the movement.  If one saw the poem in this vein, then all of the lines that were once seen as honoring could actually be flipped to be seen as final statements, eulogies read at a grave of one who has passed, as opposed to honoring the testaments of Wordsworth's contributions.  In this light, Shelley is not standing on the shoulders of giants, but rather stepping on them into the ground.